A majority of plants use environmental cues to regulate the timing of the transition from vegetative to reproductive growth in order to ensure synchronous flowering for successful outcrossing and to complete their sexual reproduction under favorable conditions (reviewed in Bernier et al., Plant Cell 5:1147-1155, 1993). The major environmental factors that control the transition are photoperiod, temperature, and nutrition.
In responding to these environmental factors, plants differ widely among species, among cultivars within species, and among stages of plant development within a cultivar. A short-day plant flowers when the day length is less than its critical length and a long-day plant flowers when the day length is longer than its critical length. Floral induction in a day-neutral species is unaffected by day-length, but occurs when the plant has attained a minimum amount of growth.
After completion of the basic vegetative phase, initiation of flowering is frequently dependent on the day length. The critical photoperiod is defined as the maximum day length that will induce a short-day plant to flower and the minimum day length that will induce a long-day plant to flower.
It has been postulated that a transmissible flowering signal is produced mainly in leaves and is transported to the shoot apex through the phloem. Grafting experiments have shown that leaves of photoperiodic plants produce promoters and inhibitors of flowering when exposed to favorable and unfavorable daylength regimes, respectively (Lang et al., Proc. Natl. Acad. Sci. USA 74:2412-2416, 1977). The nature of these transmissible signals is still controversial (O'Neill, Photochem. Photobiol. 56:789-801, 1992) and efforts to isolate the signaling substances have been unsuccessful. In addition, the target genes for these signals in the shoot apex have not been identified.
Significant effort has been expended in attempts to elucidate the underlying mechanisms controlling flower development in various dicotyledonous plant species (reviewed in Coen, Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:241-279, 1991; and Gasser, Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:621-649, 1991), leading to the isolation of a family of genes which encode regulatory proteins. These regulatory proteins include AGAMOUS (AG) (Yanofsky et al., Nature 346:35-39, 1990), APETELA I (API) (Mandel et al., Nature 360:273-277, 1992), and APETALA 3 (AP3) (Jack et al., Cell 68:683-697, 1992) in Arabidopsis thaliana, and DEFICIENS A (DEF A) (Sommer et al., EMBO J. 11:251-263, 1990), GLOBOSA (GLO) (Trobner et al., EMBO J. 11:4693-4704, 1992), SQUAMOSA (SQUA) (Huijser et al., EMBO J. 11:1239-1249, 1992), and PLENA (PLE) (Bradley et al., Cell 72:85-95, 1993) in Antirrhinum majus.
Mutations in an AG or PLE gene result in homeotic alterations of the stamen and carpel. Genetic studies have shown that the DEF A, GLO and AP3 genes are essential for petal and stamen development. API and SQUA genes, which are expressed in young flower primordia, are necessary for the transition of an inflorescence meristem into a floral meristem. Sequence analysis of these genes has revealed that their gene products contain a conserved MADS-box region (Bradley et al., Cell 72:85-95, 1993; Huijser et al., EMBO J. 11:1239-1249, 1992; Jack et al., Cell 68:683-697, 1992; Mandel et al., Nature 360:273-277, 1992; Sommer et al., EMBO J. 11:251-263, 1990; Trobner et al., EMBO J. 11:4693-4704, 1992; Yanofsky et al., Nature 346:35-39, 1990), which is probably a DNA-binding domain (Schwarz-Sommer et al., EMBO J. 11:251-263, 1992).
Using these clones as probes, MADS-box genes have been isolated from other species including tomato (Mandel et al., Cell 71:133-143, 1992), tobacco (Kempin et al., Plant Physiol 103:1041-1046, 1993), petunia (Angenent et al., Plant Cell 4:983-993, 1992), Brassica napus (Mandel et al., Cell 71:133-143, 1992), and maize (Schmidt et al., Plant Cell 5:729-737, 1993).
Transgenic approaches have been employed to study the functional roles of MADS-box genes. Genetic complementation of the ag-2 mutant by the AG gene demonstrated that the ag-2 gene product is involved in stamen and carpel development (Yanofsky et al., Nature 346:35-39, 1990). Ectopic expression of the AG genes from A. thaliana, B. napus, petunia, tobacco, and tomato resulted in homeotic conversion of sepals to carpels and petals to stamens, mirroring the ap2 mutant phenotype (Kempin et al., Plant Physiol 103:1041-1046, 1993; Mandel et al., Cell 71:133-143, 1992; Mizukami and Ma, Cell 71:119-131, 1992; Pnueli et al., Plant Cell 6:163-173, 1994; Tsuchimoto et al., Plant Cell 5:843-853, 1993). These results support the hypothesis that AG and AP2 act in an antagonistic fashion.
Antisense approaches have also been used to reveal the functional role of the tomato MADS-box genes (Pnueli et al., Plant Cell 6:175-186, 1994; Pnueli et al., Plant Cell 6:163-173, 1994). Transgenic plants that express tomato AG antisense RNA display the ag mutant phenotypes. Antisense expression of the tomato TM5 MADS-box gene results in morphological changes in the three inner whorls of transgenic plants.
The timing of the transition from vegetative growth to flowering is one of the most important steps in plant development. This determines quality and quantity of most crop species since the transition determines the balance between vegetative and reproductive growth. It would therefore be highly desirable to have means to affect the timing of this transition. The present invention meets this and other needs.